Most vehicles have engines and transmissions. The engine and transmission is used to turn a drive shaft of the vehicle or equipment. The drive shaft is connected to a differential, which transfers the rotational energy of the drive shaft to the axles and wheels of the vehicle for wheeled vehicles such as automobiles or to the axels and tracks of the vehicle for tracked vehicles such as tanks and tracked bulldozers.
Generally, when a vehicle is driving in a straight line, the tracks or wheels on the left side of the vehicle and the tracks or wheels on the right side of the vehicle rotate at the same speed. However, when the vehicle makes a turn, the tracks or wheels on the outside of the turn must travel farther than the tracks or wheels on the inside of the turn. Consequently, the tracks or wheels on the outside of the turn must rotate at a slightly faster rate than the inside tracks or wheels during the turn. The use of differentials enables opposing axles on opposite sides of the vehicle to rotate at different speeds. As such, the tracks or wheels of the vehicle can each rotate at the proper speed to accommodate a turn.
Steering is performed differently in vehicles that employ differential steering (e.g., tanks, tracked bulldozers, skid-steered loaders, etc.), than in vehicles with wheels that employ front-wheel steering. Both differentially steered vehicles and front-wheel steered vehicles share the need to permit and to control differences in wheel rotation rates between the inside and outside wheels (or tracks) during a turn. However, despite this commonality, the designs, and control methods between these two types of vehicles have several key differences, as discussed below.
In the case of a differentially-steered vehicle, a controlled difference between the left and right track speeds or left and right wheel speeds provides the sole means of executing a desired turn, and as a result, these types of vehicles require direct active control over differential rates. There are many known gear designs that aim to achieve this active control. Some designs achieve steering control by applying friction to one side of the vehicle's traction elements in order to slow down one side, thus causing the vehicle to turn toward the side that is slowed down. Such “braked” differential steering systems are inherently inefficient due to frictional losses that the propulsion system must overcome while steering. These “subtractive” designs also tend to lack precision due to the difficulty in achieving exact rotation rates through differential braking.
Other designs, such as the double differential gear, provide a separate steering input shaft to selectively alter the differential rate. Such gear-based designs are regenerative in the sense that the steering input equally accelerates one axle shaft as it slows down the opposite axle shaft. This allows greater steering authority and precision over friction-based designs. However, these gear-based differential steering designs typically employ multiple control shafts mounted adjacent to, and intermeshing with the axle shafts. For example, in the no-slip imposed steering differential described and depicted in U.S. Pat. No. 4,776,235 to Gleasman et al., these adjacent control shafts appear as elements 22 and 23 in FIG. 1. While such designs are more efficient and precise than brake-based differential steering, they typically require large and heavy housings in order to accommodate the separate drive, control, and counter-rotating idler shafts. Moreover, the need to transmit propulsive power through an increased number of counter-rotating gear elements (even during straight line travel) results in reduced drivetrain efficiency from parasitic drivetrain losses.
In the case of many conventional differentials for front-wheel steered vehicles, the difference in relative wheel rotation rates is not used to steer or turn the vehicle, but is merely accommodated (such as with an open differential) in order to preserve the vehicle's steering agility and to prevent excess tire wear and driveline binding. These conventional differentials for front-wheel steered vehicles, such as open differentials, generally work well during operation when the wheels of the automobile are encountering good road conditions. However, this approach results in a susceptibility to undesired wheel slip if the vehicle encounters slippery or unequal traction conditions. Roads are often covered in snow, ice, dirt, gravel, mud and the like that can cause a wheel to slip or skid during a turn. In such situations, open differentials may allow too much power to be applied to the slipping wheel. This can adversely affect the safety of the vehicle. In an effort to mitigate this, some differential designs employ mechanisms to detect and react to the undesired wheel slip by applying friction to slow down an already-slipping wheel. More recent advancements aim to provide more pro-active slip control such as through torque vectoring during a turn, or by pre-emptively toggling specialized traction modes for a given traction condition. Regardless, these methods still redirect power between opposing wheels by applying differential friction (either through braking or clutches) in response to steering or other inputs. This reliance on friction to limit undesired wheel slip leads to the same inherent inefficiencies and imprecisions that plague brake-based differentially-steered vehicle applications. It should be no surprise, therefore, that such friction-based slip-limiting traction control systems are often unable to arrest the loss of momentum when responding to a dramatic traction imbalance such as one wheel suddenly coming off the ground while climbing a hill.
A need therefore exists for an improved differential system for differentially steered vehicles. A need also exists for an improved differential system for front-wheel steered vehicles that precisely controls a differential rotational rate between different axle shafts while allowing for no significant slip.